The polymeric materials used in wire and cable insulation and jacketing (and other polymers) degrade with age, especially in severe environmental conditions. The safe operation of existing and future plants such as nuclear power plants requires monitoring of the insulation materials in order to anticipate degradation before performance of the wire and cable is adversely affected.
Age related wire and cable failure is primarily a mechanical mechanism. As the insulation ages, it becomes embrittled and eventually fails mechanically by cracking and exposing bare conductors. The industry has spent considerable time and effort to develop condition monitoring methods which monitor installed wire and cable and ensure that the materials have not degraded excessively. These methods are also used to predict safe operating lifetime of wire and cable insulation materials for anticipated environmental conditions. Presently, cable condition monitoring (CCM) methods are categorized as mechanical methods, chemical methods and electrical methods.
Elongation-at-break (EAB) has traditionally been one of the most common and well-documented CCM methods. This mechanical method measures the elongation of a sample of insulation material just prior to break and is normally expressed as a ratio of the break length divided by the original length of the sample. Since elongation measured in the test is analogous to elongation occurring when bending wire and cable, the results can be easily correlated to actual wire and cable insulation condition. Arrhenius methods described by others are normally used to predict material lifetime at a target ambient temperature from acceleration-aged data.
A serious disadvantage of the EAB method is that a relatively large sample portion is required to perform the test. This makes the test essentially destructive since the cable is rendered inoperative when the sample is removed. Even if a cable is sacrificed in order to run a test, some portions of the cable may be difficult or nearly impossible to access for sample removal, as would be the case if the area of interest is within a cable bundle, wire tray, or internal to a penetration. The equipment needed for measurement is relatively expensive and requires specialized skills.
Indenter modulus test is a relatively new mechanical test that utilizes a machine to press a small anvil at constant velocity against the outer surface of the cable or insulated conductor while measuring the force exerted on the anvil. The indenter modulus is defined as the slope of the force-position curve. A major advantage of the indenter modulus test is that the test itself is non-destructive. However, the test is of limited use on conductor insulation since access to a sufficient length of individual conductors is often restricted. Also, the test is not practical on cable within cable bundles or trays, or in other confined spaces.
Oxidation Induction Time (OIT) is a chemical condition monitoring method that utilizes small (8-10 mg) samples removed from cable insulation materials. The method utilizes a differential scanning calorimeter (DSC) to provide an indication of the rapid oxidation of the sample when anti-oxidants, normally present in the insulation material, are exhausted. Short induction times indicate exhaustion of the anti-oxidant and anticipate rapid degradation of the material. Sample collection requires access to the cable which limits testable portions of the cable. Measurement requires expensive laboratory equipment and specialized training.
Electrical condition monitoring methods include insulation resistance, high potential tests, tan-delta tests, and ionized gas medium tests. These tests are essentially “go-no-go” tests (do not predict the remaining life of the sample) since no well-established methods reliably predict insulation lifetime based on the results. Several of these tests require high electrical potentials to be connected to cables, requiring removal of connected equipment and loads in order for the tests to be performed.